The present invention relates to a process for introducing an insulating system inside an interspace.
Interspaces containing an insulating system have different applications, among which are Dewar flasks for the storage of cryogenic liquids, piping for the transportation of such liquids, beverage dispensing machines (wherein thermo-insulation is required mainly in order to separate the portion for the hot drinks, generally at about 70° C., from the portion for the cold drinks), containers for isothermal transportation, for example of drugs or cold or frozen food, refrigerators, and boilers.
It is commonly known for thermal insulation to use vacuum interspaces containing insulating materials, such as glass wool, colloidal silica, pearlite, and, particularly, organic polymers in the form of foams, for example open-celled rigid polyurethane.
To improve the performance of the insulating system it could be optimal to improve these insulating materials by utilization of vacuum insulating panels, known in the art by the acronym VIP.
As known, a vacuum panel is formed of an envelope inside which a filling material is present.
The envelope has the function of preventing or reducing, as much as possible, the inlet of atmospheric gases inside the panel, so as to keep a vacuum level compatible with the thermo-insulating level required by the application. For this purpose, the envelope is made with so-called “barrier” sheets, characterized by a gas permeability as low as possible, which can be formed of a single component, but more commonly of multi-layers. In this last case, the barrier effect is conferred by one of the component layers, while the other layers generally have functions of mechanical support and protection of the barrier layer.
The filling material also has the function of spacing apart the two opposite faces of the envelope when vacuum is created in the panel. During the panel evacuation, in fact, the envelope adheres to the filling material, because of the difference between the external atmospheric pressure and the internal reduced pressure of the panel. This filling material can be inorganic, such as silica powder, glass fibers, aerogels, diatomaceous earth, etc., or organic, such as rigid foams of polyurethane or polystyrene, both in the form of boards and of powders. Anyway, the filling material must be porous or discontinuous, so that the pores or the interstices can be evacuated.
Since the permeation of traces of atmospheric gases into the panel is practically unavoidable, in most cases these panels also contain one or more getter materials capable of sorbing these gases, so as to maintain the pressure inside the panel at the desired values. Preferred is the use of systems with two or three getter materials, containing at least a chemical moisture sorber and at least one component chosen among an oxide of a transition metal (having the main function of sorbing hydrogen, CO and hydrocarbons), and an alloy based on barium and lithium (having the main function of sorbing nitrogen). Various getter systems of this kind are sold by the assignee of the present application SAES Getters S.p.A. under the trademark COMBOGETTER®, among which are, in particular, systems containing a moisture sorber and alloy powder based on barium and lithium, disclosed in European Patent EP 0 769 117 B; and getter systems containing a moisture sorber and an oxide of a transition metal, with the optional addition of powder of an alloy based on barium and lithium, disclosed in European Patent EP 0 757 920 B.
Vacuum panels, and particularly those made with plastic materials, have found so far a growing use in every sector where thermal insulation is required at temperatures lower than about 100° C.
At temperatures higher than 100° C., in fact, the panel starts to deteriorate irreversibly and to release non-negligible amounts of gas, thus making it impossible to use it in all those applications whose productive processes comprise the introduction of the panel in the interspace before a heating step to be carried out at temperatures in excess of 100° C.
This limitation is onerous, since high temperature thermal treatments are often necessary, while the demand for a very good thermal insulation becomes more pressing every day.
This situation occurs unavoidably in the production of all those devices which have to be assembled before heating beyond 100° C., for example in the case of boilers.
Schematically, they are formed of an internal heating body, inserted in an external envelope, calendered and closed at its ends by two plates, the envelope having such dimensions to form with the internal body an interspace, wherein the insulating material, preferably polymeric foam, is inserted.
So far, boilers have been manufactured by a production process which can be summarized in the following way: introduction of the body of the boiler inside the external envelope, closing the ends with proper plates, thermal treatment at about 170° C. in a furnace, cooling to about 60° C., introduction into the interspace of precursors of the melted polymeric foam through apposite openings, polymerization of the precursors, and final cooling to room temperature.
The thermal treatment at 170° C. is necessary, because the external envelope of the boilers and the tops have previously been treated with powder paint, which must be desiccated in a furnace to give the coating characteristics of thermal and mechanical resistance, in particular anti-scratching properties. This operation is carried out once the structure of the boiler has been assembled, so as to minimize production times and related costs.
The need for this high temperature treatment has so far prevented insulating vacuum panels from being used in boilers, since they should be unavoidably inserted in the interspace of the boiler before closing thereof during assembly, thus before heating at high temperature.
International published patent application WO 01/51860 discloses a boiler having in the interspace an insulating system formed of a certain number of insulating panels submerged in polymeric foam, but the problems above mentioned, relative to the production process, are not solved.